Accelerated Hydrolysis of Amorphous Polylactide Containing Salicylate Additives

Blends of amorphous-grade polylactide (PLA) with low levels of salicylic acid (SA), disalicylide (DS), or oligosalicylate (OS) were prepared using scalable melt processing techniques. The glass transition temperature (Tg), tensile properties, and shelf life stability of the salicylate-containing blends were nearly identical to neat PLA. The inclusion of salicylate additives accelerated sample mass loss in artificial seawater at 50 °C by up to a factor of 3 compared to neat PLA. Water uptake occurred after the PLA molar mass dropped below 15 kg/mol, corresponding to the point at which the Tg of the remaining material dropped below 50 °C. The onset of mass loss was observed after PLA hydrolyzed into 2 kg/mol fragments, coinciding with the leaching of SA and DS; OS remained in the blends for longer immersion periods. The rate of molar-mass reduction in 1 wt % salicylate-containing blends was up to twice as fast as the rate of neat PLA. Degradation-induced PLA crystallization occurred once Tg dropped below 50 °C, and mass loss slowed after the degree of crystallinity (Xc) exceeded 50%. We conclude that a low concentration of carboxylic acid groups from salicylate-containing compounds is sufficient to enhance PLA degradability without sacrificing material performance.